Embryology: Pharyngeal Structures, GI, Ducts, Fetal Circulation

Pharyngeal Arches

• First Arch: Mandibular nerve, Meckel's cartilage, muscles of mastication, tensor tympani, mylohyoid, anterior belly of the digastric
• Second Arch: Facial nerve, Reichert's cartilage (hyoid bone), muscles of facial expression, posterior belly of the digastric, stylohyoid, stapedius
• Third Arch: Glossopharyngeal nerve, hyoid bone, stylopharyngeus muscle
• Fourth Arch: Vagus nerve, laryngeal cartilages, soft palate muscles (except tensor veli palatini), pharyngeal muscles (except stylopharyngeus)
• Fifth Arch: Vagus nerve, no specific structure, arytenoid muscle
• Sixth Arch: Vagus nerve (recurrent laryngeal), no specific structure, intrinsic muscles of the larynx (except cricothyroid and arytenoid)

Pharyngeal Pouches

• First Pouch: Auditory tube (Eustachian tube)
• Second Pouch: Tonsillar sinus, palatine tonsil
• Third and Fourth Pouches: Superior and inferior parathyroid glands, middle thymus

Gastrointestinal (GI) Development

• Esophagus: Epithelium and glands are derived from the endoderm. Striated muscle is derived from the caudal pharyngeal arches. Smooth muscle is derived from the surrounding splanchnic mesoderm.
• Stomach: Rotates 90 degrees, forming the lesser sac.
• Duodenum: As the stomach rotates, the duodenum comes to lie retroperitoneally.
• Midgut: The midgut loop rotates 90 degrees counterclockwise around the axis of the superior mesenteric artery (SMA). The cranial limb moves to the right, and the caudal limb moves to the left.
• The junction of the transverse colon is indicated by a different blood supply.

Wolffian Duct

• Male:
  • Ductus epididymis
  • Ductus deferens
  • Appendix of the epididymis
  • Seminal vesicle
  • Ejaculatory duct (appendix testis is derived from the Mullerian duct)
• Female: Gartner's duct, Gartner's cyst

Mullerian Duct

• Male: Appendix testis
• Female:
  • Sinus tubercle, hymen, seminal colliculus
  • Uterine tube, uterus (the septum should disappear; if not, it results in uterus didelphys or a bicornuate uterus), upper part of the vagina

Fetal Circulation

Highly oxygenated blood returns from the placenta in the umbilical vein. Most of it goes directly to the ductus venosus and then to the inferior vena cava (IVC). The rest goes to the hepatic sinusoids and mixes with blood from the portal circulation, also going to the IVC through the hepatic veins. After that, placental blood mixes with deoxygenated blood from the lower limbs and enters the right atrium (RA). It is directed towards the foramen ovale by the valve of the IVC.

Two paths now:

1. First (more common): Blood flows to the left atrium (LA) and mixes with a small amount of desaturated blood returning from the lungs. Then, it goes to the left ventricle (LV), ascending aorta to supply the heart, aortic arch (upper part), and descending aorta.
2. Second: A small amount of blood is prevented from entering the LA by the crista dividens and mixes there with desaturated blood from the superior vena cava (SVC) and the heart via the coronary sinus. Then, it goes to the right ventricle (RV), pulmonary trunk, ductus arteriosus (due to high pulmonary vasculature resistance), and descending aorta.

Finally, 65% of the blood passes into the umbilical arteries to the placenta for reoxygenation, and 35% supplies the viscera and inferior parts of the body.